Several methods are used to measure the distance between an apparatus and an object. Optical range-finding systems frequently rely on the time-of-flight principle and determine the distance between the apparatus and the object by measuring the time a short pulse of light emitted from the apparatus takes to reach an object and be reflected to a photo-detection circuit. Conventional optical rangefinders use a counter initiated at the starting pulse and then stopped when the receiver circuit detects the pulse echo of a value higher than a specific threshold. This threshold can be set low to provide sensitivity but the system will generate false alarms from transient noise. It can be set high to avoid false alarms but the system will not detect objects that return weak signal reflection. In bad weather conditions, such as rain or snow, several pulse echoes can be generated. Some techniques help to detect a certain number of echoes and may be used the reject some reflections but they have their limitations.
Some optical rangefinders use other methods to be more robust against false alarms. One method is based on the use of an analog-to-digital converter (ADC) for the digitalization of the waveform of the echoed back signal. Once digitalized, the waveform can be processed by digital signal processing circuits to improve the performance of the system.
Several techniques are already known for improving the performance of an optical rangefinder using an ADC. Averaging is an efficient way to improve the signal to noise ratio (SNR). However, averaging has an impact on response time and may render the system too slow for some applications.
The resolution of distance measurement can be enhanced by using a clock pulsed delay circuit technique. Using an integer (N) division of the clock pulse signal with a delay circuit and by rearranging each echo light pulse sample data, this technique improves the resolution by a factor N. However, this technique has an impact on the number of averages if the averaging technique is also used to improve the SNR.
Digital correlation is another digital processing technique for increasing the resolution of the range measurement. By correlating the echo pulse signal with a pre-stored waveform, the distance to the object can be estimated by using the peak value of the result of the correlation function.
Several digital processing techniques have been elaborated to improve the performance of rangefinders but none consider that the need, in terms of resolution and signal to noise improvement, is not constant as a function of the range for most of range-finding applications.